Multiple-Input Multiple-Output (MIMO) transmission technologies for spatially multiplexing a plurality of different data sequences (data streams) at the same frequency band and performing simultaneous communication using a plurality of antennas for transmission and reception have been put into practical use in wireless LANs or cellular systems. In Single User MIMO (SU-MIMO) of spatially multiplexing a plurality of different data sequences and transmitting the data sequences to one certain terminal apparatus (a reception apparatus or user equipment (UE)), there are methods of precoding transmission signals and transmitting the transmission signals in a base station apparatus (a transmission apparatus, an eNodeB, or an access point) are used to improve performance of separation and detection of the plurality of data sequences in a terminal apparatus.
In cellular systems of Long Term Evolution (LTE), LTE-Advanced (LTE-A), or the like standardized in Third Generation Partnership Project (3GPP) or wireless LAN systems of IEEE 802.11ac or the like standardized in the Institute of Electrical and Electronics Engineers, Inc (IEEE), systems in which the number of transmission antennas included in a base station apparatus (access point) is considerably greater than the number of reception antennas included in a terminal apparatus have been proposed. Further, in order to further improve system throughput by effectively utilizing a plurality of transmission antennas of a base station apparatus, Multi-User MIMO (MU-MIMO) for performing MIMO multiplexing on data sequences destined for a plurality of terminal apparatuses (users) has been proposed.
In MU-MIMO, since a transmission signal destined for another terminal apparatus is received as inter-user interference (IUI) in a terminal apparatus, it is necessary to suppress the IUI. Several methods have been proposed in which transmission signals capable of suppressing occurrence of IUI can be generated at the time of reception in terminal apparatuses without applying a large load to the terminal apparatuses when base station apparatuses know channel states from each transmission antenna of the base station apparatus to each reception antenna of each terminal apparatus (see NPL 1).
For example, in LTE-A, beamforming (linear precoding) is adopted in which each terminal apparatus selects one precoding matrix from among candidates (codebooks) of the precoding matrix based on a channel state and notifies a base station apparatus of the index (Precoding Matrix Indicator (PMI)), and the base station apparatus suppresses IUI by multiplying a transmission signal in advance by a linear filter (transmission weight) calculated based on the PMI in the base station apparatus (see NPL 2).
However, IUI cannot be efficiently suppressed unless orthogonality of a channel subjected to spatial multiplexing between terminal apparatuses is high. Therefore, there is a limit to an improvement in frequency use efficiency in MU-MIMO based on linear precoding (linear MU-MIMO).
Accordingly, in recent years, MU-MIMO (nonlinear MU-MIMO) technologies in which nonlinear precoding is performed on the side of a base station apparatus have been noticed. When modulo (surplus) calculation is possible, a terminal apparatus can add a perturbation vector, which has a complex number (perturbation term) obtained by multiplying any Gaussian integer by a real number as a component, to a transmission signal. Accordingly, by appropriately setting a perturbation vector according to channel states between a base station apparatus and a plurality of terminal apparatuses and generating transmission signals, necessary transmission power can be considerably reduced more than linear precoding in which no perturbation vector is added, for example, even when orthogonality of a channel subjected to spatial multiplexing between terminal apparatuses is not high. Thus, transmission efficiency can be considerably improved (see NPL 3).